1. Field of the Invention
The present invention relates to an image forming method, an image forming apparatus, and a toner cartridge, which are used when forming during formation of an image by developing an electrostatic latent image with a method such as an electrophotographic method or an electrostatic recording method.
2. Description of the Related Art
Formation of an image using an electrophotographic method is performed by developing an electrostatic latent image formed on the surface of a latent image-carrier (i.e., photosensitive member), with a toner containing a colorant. The resulting toner image is transferred onto a transfer receiving material such as paper and fixed with a device such as a thermal roll. The surface of the latent image-carrier is generally cleaned after transfer of the toner image in order to form again an electrostatic latent image.
A dry developer used in such the electrophotographic method is roughly classified into a one-component developer which uses a toner obtained by incorporating a colorant and the like into a binding resin alone, and a two-component developer obtained by mixing a carrier into a toner. The one-component developer can be classified into a magnetic one-component developer which uses a magnetic powder, and is conveyed by a developer-carrier with a magnetic force, followed by developing, and a non-magnetic one-component developer which does not use a magnetic powder, and is conveyed by a developer-carrier with electrification impartment, followed by developing.
Since the late 1980s, in the market of electrophotography, miniaturization and high functionalization have been strongly demanded targeting digitalization and, in particular, regarding the full color image quality, high grade printing, and high image quality class near that of a silver halide photography are desired. As means for attaining the high image quality, digitalization treatment is essential. As the efficacy of digitalization regarding such the image quality, there is complex image treatment at a high speed. For that reason, in a digital format, it becomes possible to control letter images and photography images separately, and the reproductivity of letter images and photography images has been greatly improved as compared with an analog format. In particular, regarding a photography image, since gradation correction and color collection have become possible, a digital format is advantageous in the gradation property, the fineness, the sharpness, the color reproductivity and the granularity as compared with an analog format.
Upon image formation, it is necessary to faithfully reproduce a latent image produced by an optical system as an image. For this reason, for the purpose of faithfully reproducing an image, research and exploitation for making a particle diameter of a toner smaller are being performed more actively. However, only by merely making a particle diameter of a toner smaller, it is difficult to stably achieve a high quality image, and improvement in fundamental property regarding development, transfer and fixation has become more important.
When a color image is obtained, generally, three color or four color toners are overlaid to form an image. For that reason, when any of these color toners exhibits the different property from the initial property or the different performance from that of other color from a viewpoint of development, transfer and fixation, reduction in the color reproductivity, deterioration in the granularity, and deterioration in the image quality such as uneven color are caused. In order to maintain an initial high quality image stably and for a long term, how the property of each color toner is stably controlled, is important.
In recent years, from a viewpoint of speeding up when a color image is obtained (referred to simply as “color speeding up” in some cases), a so-called tandem developing system-type image forming apparatus using a plurality of developing units composed of a developing device containing a developer-carrier, a latent image-carrier and the like, is adopted. In the tandem developing system, from a viewpoint of complying to space saving and miniaturization of an image forming apparatus, a small diameter of a latent image-carrier in each developing unit is sought. Regarding such the tandem developing system, many studies have been made (e.g. Japanese Patent Application Laid-Open (JP-A) Nos. 6-35287 and 6-100195).
By adopting such the tandem developing system, color speeding up becomes easier as compared with the rotary developing system. However, in the tandem development system, even when a single color image such as black is formed, it is general that other color developer-carriers are also in contact with a latent image-carrier and, at the same time, are forced to rotate in a process direction.
In such the case, since a stress received by a developer is large, and reduction in the electrifiability of a developer is induced, reduction in the developing performance and reduction in the transferring performance are easily caused, and finally, leading to deterioration in the image quality. In addition, in the tandem developing system, since a size of one developing device is limited due to limitation of a space around a latent image-carrier, or a size of an apparatus, a sufficient amount of a developer can not be stored in each developing device. Therefore, a stress received by a developer is liable to be greater due to a structure of such the apparatus. For that reason, exchange of a developer is frequently performed with deterioration in a developer, and this leads to remarkable increase in the service cost.
As means for suppressing deterioration of a developer, JP-A No. 8-234550 proposes the technique using several kinds of supplementary developers containing carriers having the different physical properties. However, in this technique, since variation in the physical property of a carrier has influence on the toner flowability, the properties between toner colors and the like, control system becomes complex, leading to scaling up or increase in the cost of an apparatus.
In addition, JP-A No. 11-202636 proposes the technique of supplementing a supplementary developer containing a carrier having a larger amount of electricity than that of a carrier used in a starting developer. This technique is very advantageous in prolonging a life of a developer. On the other hand, when the image stability is taken into consideration, it is important that the physical property of a developer does not change by the environment and continuous use, but this technique controls the physical property of a developer microscopically with difficulty.
As a developer, a two-component-developer composed of a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner alone is known. For preparing the toner, a kneading and grinding process is usually utilized in which a thermoplastic resin is melted and kneaded together with a pigment, a charge control agent, a releasing agent such as a wax, which is cooled, and finely-divided and classified. If needed, in order to improve the flowability and the cleanability, a fine particle composed of an inorganic material or an organic material is added to the surface of the toner in some cases. A toner prepared by utilizing these processes for preparing a toner has the most excellent property, but has some problems as described below.
For example, when a toner is prepared by a kneading and grinding process, a shape and a surface structure of the resulting toner are undefined. In addition, although a shape and a surface structure of a toner are subtlety changed depending on the grindability of a material used as a raw material and the conditions of a grinding step, it is difficult to intentionally control a shape and a surface structure of a toner. In addition, in a kneading and grinding process, there is a limitation on a range of material selection. Specifically, a dispersion of a colorant in a resin in which a colorant is dispersed in a resin must be sufficiently brittle and can be finely-divided with an economically available preparing apparatus. However, when a dispersion of a colorant in a resin is made to be brittle in order to satisfy such the request, a fine powder is produced by a mechanical shearing force applied to a toner in a developing device, and a toner shape is changed in some cases.
Due to these influences, in a two-component developer, deterioration in electrification of a developer due to adhering of a fine powder to the carrier surface is accelerated and, in a one-component developer, scattering of a toner is caused due to expansion of a particle size distribution, and deterioration in the image quality is easily caused due to reduction in the developability by a change in a toner shape.
In addition, when a toner is prepared by internally adding a large amount of a releasing agent such as a wax, a releasing agent is remarkably exposed on the toner surface depending on a combination of a thermoplastic resin and a releasing agent. In particular, in a combination of a resin, which has the increased elasticity due to a high-molecular component and is slightly ground with difficulty and a brittle wax such as polyethylene, exposure of polyethylene on the toner surface is observed frequently. Such the exposure of a releasing agent on the toner surface is advantageous in the releasability at fixation and cleaning of an untransferred toner remaining on the surface of a photosensitive member. However, since a releasing agent exposed on the toner surface is easily transferred to another member by a mechanical force, contamination of a developing roll, a photosensitive member and a carrier is easily caused, leaving to reduction in reliance.
Further, when a toner shape is undefined, even if a flowing assistant is added, the sufficient flowability is not retained in some cases. In such the case, a fine particle present on the toner surface is transferred to a recess part on the toner surface by a mechanical shearing force applied to a toner at image formation, whereby, the flowability of a toner is reduced with time, and a flowing assistant is embedded in the interior of a toner, whereby, the developability, the transferability and the cleanability are deteriorated. In addition, when a toner collected by cleaning is returned again to a developing device, and is used, deterioration in the image quality is further easily caused. When an amount of a flowing assistant to be added to a toner is further increased, a black point is produced on the surface of a photosensitive member, and scattering of a flowing assistant particle is caused.
In recent years, as means that allows for intentional control of a shape and a surface structure of a toner, JP-A Nos. 63-282752 and 6-250439 propose a process for preparing a toner by a polymerization aggregating method. This process for preparing a toner is generally a process for preparing a toner by mixing a resin fine particle dispersion prepared by emulsion polymerization and a colorant dispersion prepared by dispersing a colorant in a solvent, to form an aggregate having a diameter equivalent to a toner particle diameter, then, heating this aggregate to melt and coalescing it. The toner obtained by this process is not only easy in downsizing a diameter of a toner, but also extremely excellent in a particle size distribution.
In more recent years, a demand on the higher image quality is increased and, in particular, in color image formation, in order to achieve a highly fine image, there is a remarkable tendency that a toner to be used is downsized. However, when a toner is simply downsized while maintaining the previous particle size distribution, since toners on a smaller diameter side of the particle size distribution are present, contamination of a carrier and a photosensitive member and scattering of a toner increases remarkably, and it is difficult to achieve the high image quality and the high reliance at the same time. For that reason, a toner having a sharp particle size distribution and a small particle diameter is necessary. From a viewpoint that such the toner can be obtained, an emulsion polymerization aggregation method is advantageous as a process for preparing a toner.
In addition, recently, from a viewpoint of conversion into a digital machine and improvement in the productivity of office documents, when speeding up and energy saving are taken into consideration, a toner is also required to have the fixability at a lower temperature. Also from these points, a toner having a sharp particle size distribution and which is prepared by an aggregating and coalescing method suitable for preparing a toner has the excellent property.
In addition, a method of covering the surface of a fixing member such as a fixing roll with a fluorine series resin film such as polytetrafluoroethylene for the purpose of reducing the wettability between a developed and transferred toner and a material such as a paper, and maintaining a peelability can be utilized as one means for attaining the aforementioned energy saving. However, since this fluorine series resin film inhibits conduction of thermal energy supplied to a fixing roll in some cases, a thickness of the film is limited. In addition, when a thickness of the fluorine series resin film is decreased for the purpose of making influence on thermal conduction small, the durability of a fixing member is deteriorated in some cases due to remarkable occurrence of crease on the surface of a fixing member. For this reason, there is desired exploitation of a toner, which is not necessary to cover the surface of a fixing member such as a fixing roll with a fluorine series resin film.
Further, variation in a toner shape, and a particle diameter and the flowability of a toner produces variation in electrification of a toner, and the insulating property of a toner has influence on the electrification maintaining property. Such the variation in the physical property of a toner leads to occurrence of phenomenon (so-called selective developing phenomenon) in which a toner having the better electrifiability is selectively consumed upon image formation, a toner having the low electrifiability remains in a developing device, and deterioration in the developability is caused as a whole developer.
When deterioration of a developer is accelerated due to selective developing, it becomes necessary to exchange a developer, leading to remarkable increase in the service cost. In particular, in the tandem developing system, since a sufficient amount of a developer can not be stored in each developing device from a viewpoint of a space, deterioration of a developer is easily accelerated due to variation in the electrifiability of a toner and, thus, there is desired improvement in the maintenance of a developer also from a viewpoint of a toner.
In addition, JP-A No. 10-312089 reports that, by stirring a toner in a developing device, a microstructure of the toner surface is easily changed, and the transferability is greatly changed. By a change in a microstructure of the toner surface, variation in the electrifiability of a toner easily becomes large, resulting in promotion of selective developing, and reduction in maintenance of a developer becomes more remarkably problematic.